Foxtrotalpha reports there may be reason to believe that the ship was torpedoed rather than having been destroyed by an ammunition loading accident.

I considered that it might have been an attack carried out by Kaiten, submarine launched manned suicide torpedoes. They were being used at that time to attack shipping in forward bases. Kaiten might have made an attack on a protected harbor easier, but the link in this paragraph provides a listing of operations that seems to preclude that possibility. That in spite of the fact that there were about 20 Kaiten capable Japanese submarines operational at the time of the sinking.

No evidence to suggest that it was a Kaiten, just that other Kaiten operations were attacking ships in port during this period. We had other ammunition handling explosions including Port Chicago and at least one other in WWII in addition to the Serpens.

In 1945 USS South Dakota (BB-57) suffered an explosion in Turret 2 during ammunition loading at a forward base as well. Every day of late WWII there were millions and millions of bombs, rockets, shells, propellant, and ammunition being handled. Accidents are bound to happen, just from the law of averages…